1. Field of the Invention
The present invention relates to a spread spectrum frequency-hopping multilevel frequency shift keying (FH-MFSK) receiver and, more particularly, to a spread spectrum L-length frequency-hopping sequence, Q-level frequency shift keying receiver which is capable of decoding an L-length received signal using a particular user's FH address sequence and using a unique checking technique for more accurately choosing the correct one of one or more of the possible received Q levels.
2. Description of the Prior Art
Spread spectrum radio communication systems using various modulation techniques have evolved to provide protection of transmitted radio signals from detection, demodulation and/or interference by outside sources. In multiple access spread spectrum communication, each user has access to the whole system bandwidth. One way of distinguishing the signals from different users is to give each user an address consisting of a fixed pattern in time and frequency. The information to be transmitted is modulated or coded onto the address. The receiver detects the appropriate address and decodes the message. This technique is often referred to as random-access-discrete-address (RADA) or code-division multiple access (CDMA).
The two major impairments of mobile radio communication systems are interference from other users and multipath fading. The conventional Frequency Division Multiple Access (FDMA) technique uses guard bands between frequency channels to minimize interference and increased signal power to combat fading. Recently, various frequency-hopping (FH) techniques havr been proposed for use in mobile and satellite radio systems.
One such technique is disclosed in the article "A Spread Spectrum Technique for High Capacity Mobile Communications" by G. R. Cooper et al. in the Conference Record of the Twenty-Seventh Annual IEEE Vehicular Technology Conference at Orlando, Fl. on Mar. 16-18, 1977 at pages 98-103 which relates to a cellular spread spectrum frequency-hopped, differential phase shift keying (FH-DPSK) mobile communication system. In a typical receiver an array of delay lines and bandpass filters selects the desired address waveform out of the incoming signal. The phases of the various elements of the desired signal are detected relative to the previous word and are then passed through a linear combiner and maximum-likelihood decision circuit for proper processing.
Another technique for use with a satellite communication system is disclosed in the article "A Processing Satellite Transponder for Multiple Access by Low-Rate Mobile Users" by A. J. Viterbi in the Conference Record of the Fourth International Conference on Digital Satellite Communications at Montreal, Canada on Oct. 23-25, 1978 at pages 166-174 which relates to a frequency-hopping, multilevel frequency shift-keyed (FH-MFSK) arrangement. In the disclosed receiving section, a set of received Q=2.sup.K level, L-length signals are spectrally decoded and processed to produce an L-length transmission in the decoded signal. Usually only one such signal is produced. If two or more L-length signals are provided, resolution would be to arbitrarily select one as the correct message.
The use of FH-MFSK techniques with mobile radio communication systems, however, provides problems of, for example, multipath fading and noise which are not necessarily as prevalent in satellite communication systems. Typically, multipath propagation and noise would lead to imperfect tone detection of the various concurrent frequency-hopped signals at the receiver, causing additional errors. In some instances, multiple full L-length levels may be found in decoding a received signal of many concurrent users. At other instances the degrading of certain signals by multipath propagation and noise may result in no full L-length signal sequence being found at the receiver, in which case the longest of the signal sequences would be selected.
The problem remaining in the prior art is to provide a FH-MFSK received which can decode multiple concurrently received FH-MFSK signals and provide an output signal indicative of the correct signal under conditions where the decoding results in one or more sequences which share a maximum length equal to or less than L or some other predetermined maximum obtainable value.